As a conventional X-ray CT (computed tomography) system, there is an X-ray CT system that detects X-rays radiated from an X-ray tube onto a subject and transmitted through the subject by means of an X-ray detector, acquires projection data, and reconstructs an image from the acquired projection data.
FIG. 14 illustrates imaging regions. As illustrated in FIG. 14, the radiated X-rays have a cone angle θ such that they spread from the X-ray tube in the body-axis direction of the subject (cone-beam). The imaging regions (Field OF View: FOV) in the subject are provided within the cone angle θ.
A step and shoot type of scan is used in which the imaging regions in the subject are displaced and imaged by radiating the cone-beam onto the subject every time a top is moved in the body-axis direction by a predetermined transfer amount. In addition, such a scan is sometimes referred to as a wide-volume scan.
In the event of considering such a scan, reducing the region overlapped between the adjacent imaging regions and composed upon reconstruction as much as possible contributes to the reduction of radiation exposure. FIG. 14 illustrates the adjacent imaging regions as a region surrounded by bold solid lines and a region surrounded by bold broken lines. Moreover, when the shape of the adjacent imaging regions are both hexagons, the range of the overlap region when imaging is represented as β.
However, different from a helical scan, the more the overlap region is reduced, the worse the continuity between the imaging regions becomes, resulting in the sharp appearance of discontinuity in the imaging region boundary. In order to avoid this, discontinuity in the boundary is obscured by providing an overlap region and carrying out scanning, then smoothly shifting the overlap region (feathering).
However, conventional sagittal/coronal images become hexagons (refer to FIG. 14). Consequently, more projection data for the overlap region must be provided than necessary such that feathering is easily carried out by sufficiently using the projection data of the overlap region. On the other hand, recently, it has become possible to image regions that could not be reconstructed, making it possible to reconstruct a wider range from the projection data of one imaging region. Accordingly, in principle, it has become possible to minimize the overlap region.
However, according to an example of a conventional X-ray CT system for scanning a heart, when dividing the heart into two imaging regions, because the heart is always beating, there are many cases in which the shape of the heart is different in a first imaging region and a second imaging region. In such a case, if the overlap region is minimized, discontinuity in the imaging region boundary sharply appears to deteriorate the image quality.
In another example, the top board has a problem in terms of rigidity. Because the bend amount of the top board cannot be reduced to zero, the larger the range of one imaging region and the longer the distance between the imaging regions, the larger the difference in the bend amounts of the top board becomes; moreover, if the overlap region is minimized, discontinuity in the imaging region boundary sharply appears to deteriorate the image quality.
In addition, according to X-ray detectors of recent years, the X-ray detecting elements are arranged in multiple rows in the body-axis direction. Consequently, the distance between the imaging regions tends to be long, resulting in a problem of discontinuity in the imaging region boundary.